1. Field of the Invention
This invention is related to the field of electronic systems and, more particularly, to providing a supply voltage to an integrated circuit in an electronic system.
2. Description of the Related Art
As the number of transistors included on a single integrated circuit “chip” has increased and as the operating frequency of the integrated circuits has increased, the management of power consumed by an integrated circuit has continued to increase in importance. If power consumption is not managed, meeting the thermal requirements of the integrated circuit (e.g. providing components required to adequately cool the integrated circuit during operation to remain within thermal limits of the integrated circuit) may be overly costly or even infeasible. Additionally, in some applications such as battery powered (e.g. portable) devices such as laptops, personal digital assistants (PDAs), mobile phones, etc., managing power consumption in an integrated circuit may be key to providing acceptable battery life.
For those devices that include a processor, or central processing unit (CPU), a common power management technique is to power down the processor if there are no computation requirements for the processor. In the powered-down state, the supply voltage has been deactivated (or “turned off”). However, when the user of the device requires an operation that involves the processor, the response time must be very fast to avoid the appearance of low performance to the user. To support a rapid response to user input from the powered-down state, the processor's supply voltage must rise rapidly (e.g. in the range of 1-10 microseconds) so that the processor can start executing instructions. Increasing the magnitude of the supply voltage (e.g. from ground to the specified voltage magnitude for the processor) is also referred to as “ramping” the supply voltage.
Currently, DC-DC converters are typically used in mobile devices to provide rapid ramp of the supply voltage. The efficiency of DC-DC converters is typically high, but the size and cost of the devices involved in a fast converter is often prohibitive for cost sensitive applications or volume sensitive applications. The ramp time is generally limited by the amount of bypass capacitance required by the CPU to operate and by the size of the transistors used in the switcher of the DC-DC converter. The efficiency of the DC-DC converter depends on the switching losses in the transistor (i.e. the larger the transistor, the higher the loss). To provide a fast ramp time, the converter analog circuits need to have a wide bandwidth and operate at fast switching frequencies, which increases the losses. On top of that, the circuits are typically high order circuits that cause overshoot in the supply voltage. The overshoot can damage the CPU.